1. Field of the Invention
This invention relates to a Manchester type carry propagation circuit. More particularly, this invention relates to a Manchester type carry propagation circuit for propagating a carry generated in performing operations by an arithmetic logic unit.
2 Description of the Prior Art
A Manchester type carry propagation circuit for propagating carry generated in performing operations by an arithmetic logic unit is known by Carver Mead et al. "Introduction to VLSI Systems", ADDISON WESLEY.
FIG. 1 is an electric circuit showing major portions of a precharge section of a bit of the prebeding stage and a carry propagation section of the succeeding stage. First, a conventional Manchester type carry propagation circuit will be described with reference to FIG. 1. Referring to FIG. 1, the NMOS transistor 1 is to precharge carry signal line 5. The NMOS transistor 2 turns on/off in response to a carry propagation signal 6, and the NMOS transistor 3 turns on/off in response to a carry clear signal 8. The NMOS transistor 4 works in response to the carry propagation signal 7, to propagate a carry of the preceding stage to the succeeding one.
When a precharge clock signal .phi.1 is applied to the gate of the NMOS transistor 1, this NMOS transistor 1 becomes conductive and precharges the carry signal line 5. And when the carry propagation signal 6 becomes "L", the NMOS transistor 2 becomes non-conductive, and the signal line 7 becomes "H". In response to this, the transistor 4 becomes conductive, and carry 10 of the preceding stage is propagated across the carry signal line 5 to the succeeding stage. In this Manchester type carry propagation circuit shown in FIG. 1, the carry signal line 5 is adapted to be precharged before the NMOS transistor 2 becomes conductive, so that the carry "H" can be transmitted to the succeeding stage without time delay.
In propagating a carry of "L" level to the succeeding stage, the transistor 1 becomes conductive by the precharge clock signal .phi.1, as described above, and precharges the carry signal line 5. When the carry propagation signal 6 becomes "L", the NMOS transistor 2 becomes non-conductive, the signal line 7 becomes "H", and the carry propagating transistor 4 becomes conductive. Due to the fact that this transistor 4 became conductive, electric potential of the carry signal line 5 becomes "L" gradually.
Thus, in the conventional Manchester type carry propagation circuit, the "H" level of the carry 10 need not be propagated, as the carry signal line 5 has reached the "H" level by the time when the carry 10 of the preceding stage reaches the "H" level. However, when the carry 10 is at the "L" level, that level is propagated to the carry signal line 5 through the carry propagating transistor 4. In this case, the carry signal line 5 has attained the "H" level (5V) during the precharge period, and if the carry 10 of the preceding stage reaches "H" in this condition, the carry signal line 5 begins to discharge. At this time, even if the carry propagation signal 6 reaches the "H" level, the carry propagating transistor 4 does not become conductive until the voltage at the carry signal line 5 becomes lower than the gate voltage of the carry propagating transistor 4 by more than the threshold voltage Vth of this transistor. Therefore, a period of time after the carry propagation signal 6 reached the "L" level until the carry propagating transistor 4 becomes conductive becomes wasteful, and even if all bits of the carry propagation signal 6 are set simultaneously, a substantial time is required until the level of the carry signal line 5 becomes the final carry propagation output, resulting in a disadvantage that the speed of operation slows down.